When samples are imaged or processed with a particle beam microscope, such as an electron microscope, they are usually kept in a vacuum environment in the specimen chamber. The specimen chamber is evacuated by a vacuum pump. Typically, measurements with scanning electron microscopes are conducted at a vacuum level in the specimen chamber in a range of about high vacuum and about 22.5 Torr. The specimen chamber is therefore designed as a vacuum vessel, having solid walls and flanges, such that the leaking rates of atmospheric leaks can be kept as low as possible. Hence, the vacuum vessel usually does not have windows, which are large enough to allow a user to control the positioning of the object in front of the objective lens by visual observation.
Typically, the positioning of the sample is monitored by a COD-camera, which is arranged within the specimen chamber. The camera acquires a video image from the sample and the objective lens, which is displayed on a display. By looking at the video image, the user can observe the positioning process in real time and control the positioning of the sample via control signals, which are transmitted to a positioning device.
However, the displayed video image provides the user only with a two dimensional image from the interior of the specimen chamber, such that it is complicated to accurately position the object relative to the object lens. Furthermore, the viewing angle of the COD-camera for observing the object surface is typically obstructed by the objective lens and detectors, especially when the object is located close to the objective lens. Hence, the user quite often is not able to determine, which part of the sample is irradiated by the electron beam.
Beside the objective lens, there are typically also further components arranged in the interior of the specimen chamber, which may obstruct the view to the sample during a positioning process. Examples of such components are detectors, gas injection systems and manipulators. These components may also collide with the sample during a positioning process.
Conducting the positioning is even more complicated when a number of objects, in particular objects having a complex geometry, are attached to the object holder for being positioned in front of the objective lens.
By an inaccurately conducting the positioning process, it is possible that collisions occur, which may result in damages to either the object or to components of the electron microscope.
It has been recognized, that the positioning of a sample inside of a particle beam microscope is complicated to conduct. Thereby, handling the particle beam microscope for carrying out a positioning process within, a reasonable amount of time requires a lot of experience.